In electrical distribution networks, it is necessary to protect equipment connected along the distribution network from damage which may be introduced by power or voltage surges from lightning or voltage overloads. This is often accomplished by the insertion into the system of a surge arrester. A surge arrester is an electrical device whose function is to protect electrical power distribution systems from overvoltages due to lightning, switching surges, and temporary power frequency overvoltages due to line-to-ground faults, ferroesonance, etc. Present day surge arresters generally consist of voltage non-linear elements, commonly called valve elements, enclosed in one or more housings made of porcelain, fiber-reinforced materials, polymeric resins, and the like. Said voltage non-linear elements may include spark gaps alone and/or in combination with valve elements made of silicone carbide (SIC), zinc oxide (ZnO), titanium dioxide, or strontium titanate. Recent surge arrester designs utilize ZnO valve elements without spark gaps, so-called gapless arresters.
The surge arrester is commonly attached to the electrical distribution system in a parallel configuration, with one end of the device connected to the electrical system and the other end connected to ground. At normal system voltages, the surge arrester is electrically resistant to current flow. However, if an overvoltage condition occurs, the surge arrester becomes conductive and shunts the surge energy to ground while "clamping" or limiting the voltage to an acceptable value. In this manner, the surge arrester protects other equipment attached to the system from the possibly deleterious effects of overvoltage surges.
Surge arresters were originally made with heavy porcelain housings that made them cumbersome to install and subject to breakage. Later improvements included semiconductive varistor valve elements such as doped ZnO, polymeric plastic sheds or housings and composite internal structural members. Recent advances in surge arrester design and products have focused on primarily four areas.
Polymeric structural members and housings have been used outside the valvo and terminal elements. These housings are less heavy than prior ceramic housing and also less fragile. However, these housings are not vented and problems with explosive fragmentation can occur.
Other advances have focused both on eliminating the cause of arrester failures as well as reducing the hazards of failure. Failure is often caused by degradation of the valve elements and device through the ingress of moisture. A second area of recent improvement is interface sealing between the outer housing and the structural element, or terminal element, to avoid gross areas of moisture ingress. An example is illustrated in U.S. Pat. No. 4,851,955.
Another type of moisture ingress, diffusion through the housing materials, can occur in a completely sealed design. This moisture diffusion problem is addressed with a void-free design. However, this design may also fragment during a failure event.
The fragmentation problem was addressed with a vented fiberglass, structural, member where the gases escape during a failure event through slits in a tubular housing. This is illustrated in U.S. Pat. No. 4,930,039 the disclosure of which is incorporated herein by reference for all purposes, as well as Japanese disclosure S63(1988)-312602 of Dec. 21, 1988.
Manufacturing a device which requires insertion of a valve element into the tubular outer structural member and sealing it to ensure that it is void free is an exceedingly complex, time consuming and difficult task, if achievable at all. Providing fragmentation explosion resistance with venting in a sealed, void free unit is a complex problem. Satisfying all these requirements in a design which provides ease of manufacturability raises even more complex issues.
Thus it would be highly desirable to have a sealed void free but venting surge arrester which can be manufactured in a simple and straight forward process with a minimum of complex assembly operations.